/*
 * Copyright (c) 2003, 2005, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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 *
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 *
 *
 *
 *
 *
 *
 */


package com.sun.java_cup.internal.runtime;

import java.util.Stack;

/**
 * This class implements a temporary or "virtual" parse stack that
 * replaces the top portion of the actual parse stack (the part that
 * has been changed by some set of operations) while maintaining its
 * original contents.  This data structure is used when the parse needs
 * to "parse ahead" to determine if a given error recovery attempt will
 * allow the parse to continue far enough to consider it successful.  Once
 * success or failure of parse ahead is determined the system then
 * reverts to the original parse stack (which has not actually been
 * modified).  Since parse ahead does not execute actions, only parse
 * state is maintained on the virtual stack, not full Symbol objects.
 *
 * @author Frank Flannery
 * @see com.sun.java_cup.internal.runtime.lr_parser
 */

public class virtual_parse_stack {
  /*-----------------------------------------------------------*/
  /*--- Constructor(s) ----------------------------------------*/
  /*-----------------------------------------------------------*/

  /**
   * Constructor to build a virtual stack out of a real stack.
   */
  public virtual_parse_stack(Stack shadowing_stack) throws java.lang.Exception {
      /* sanity check */
    if (shadowing_stack == null) {
      throw new Exception(
          "Internal parser error: attempt to create null virtual stack");
    }

      /* set up our internals */
    real_stack = shadowing_stack;
    vstack = new Stack();
    real_next = 0;

      /* get one element onto the virtual portion of the stack */
    get_from_real();
  }

  /*-----------------------------------------------------------*/
  /*--- (Access to) Instance Variables ------------------------*/
  /*-----------------------------------------------------------*/

  /**
   * The real stack that we shadow.  This is accessed when we move off
   * the bottom of the virtual portion of the stack, but is always left
   * unmodified.
   */
  protected Stack real_stack;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /**
   * Top of stack indicator for where we leave off in the real stack.
   * This is measured from top of stack, so 0 would indicate that no
   * elements have been "moved" from the real to virtual stack.
   */
  protected int real_next;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /**
   * The virtual top portion of the stack.  This stack contains Integer
   * objects with state numbers.  This stack shadows the top portion
   * of the real stack within the area that has been modified (via operations
   * on the virtual stack).  When this portion of the stack becomes empty we
   * transfer elements from the underlying stack onto this stack.
   */
  protected Stack vstack;

  /*-----------------------------------------------------------*/
  /*--- General Methods ---------------------------------------*/
  /*-----------------------------------------------------------*/

  /**
   * Transfer an element from the real to the virtual stack.  This assumes
   * that the virtual stack is currently empty.
   */
  protected void get_from_real() {
    Symbol stack_sym;

      /* don't transfer if the real stack is empty */
    if (real_next >= real_stack.size()) {
      return;
    }

      /* get a copy of the first Symbol we have not transfered */
    stack_sym = (Symbol) real_stack.elementAt(real_stack.size() - 1 - real_next);

      /* record the transfer */
    real_next++;

      /* put the state number from the Symbol onto the virtual stack */
    vstack.push(new Integer(stack_sym.parse_state));
  }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /**
   * Indicate whether the stack is empty.
   */
  public boolean empty() {
      /* if vstack is empty then we were unable to transfer onto it and
         the whole thing is empty. */
    return vstack.empty();
  }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /**
   * Return value on the top of the stack (without popping it).
   */
  public int top() throws java.lang.Exception {
    if (vstack.empty()) {
      throw new Exception(
          "Internal parser error: top() called on empty virtual stack");
    }

    return ((Integer) vstack.peek()).intValue();
  }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /**
   * Pop the stack.
   */
  public void pop() throws java.lang.Exception {
    if (vstack.empty()) {
      throw new Exception(
          "Internal parser error: pop from empty virtual stack");
    }

      /* pop it */
    vstack.pop();

      /* if we are now empty transfer an element (if there is one) */
    if (vstack.empty()) {
      get_from_real();
    }
  }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /**
   * Push a state number onto the stack.
   */
  public void push(int state_num) {
    vstack.push(new Integer(state_num));
  }

  /*-----------------------------------------------------------*/

}
